docs: def build_loss_maps(self, losses, clp, stats=()):
          """
          :param losses: an array of shape R, E
          :param clp: a list of C conditional loss poes
          :param stats: list of pairs [(statname, statfunc), ...]
          :returns: two arrays of shape (C, R) and (C, S)
          """
          array = numpy.zeros((len(clp), len(losses)), F32)
          for r, ls in enumerate(losses):
              if len(ls) < 2:
                  continue
              for c, poe in enumerate(clp):
                  array[c, r] = conditional_loss_ratio(ls, self.poes, poe)
          return self.pair(array, stats)